Sandwich panels are used in a wide variety of applications requiring structural and/or thermal insulation properties. These applications include structural and non-structural uses in refrigerated and non-refrigerated buildings, boats, aircraft, rapid transit and recreational vehicles, and enclosed trailers. Structural panels are composite structures formed by bonding two generally thin facings or skins to a relatively thick core material. The skins, which are normally dense and strong, resist compression and tension, while the core, which is normally of relatively weak and low-density material, serves to separate the skins, stabilize them against buckling and resist shear loads.
Two common materials used as cores for sandwich panels are rigid expanded plastic foams and honeycomb materials. The honeycomb core usually comprises a thin sheet material such as paper which is formed into a variety of cellular configurations. The expanded plastic foam cores usually provide much higher levels of thermal insulation than honeycomb, but the honeycomb cores are normally substantially stronger than the insulating foam cores.
Plastic foam core sandwich panels often involve serious compromises in their design and cost due to inherent structural limitations of the rigid foam insulation cores. In addition to the deflection of these panels due to compressive and tensile stresses in the skins, further deflection results from the low shear modulus of the rigid foam material. The thicker the core, the more important shear deflection becomes, to the point of exceeding deflection due to bending. Under a sustained load, the plastic foam core is subject to creep deformation, further increasing panel deflection, with resulting risk of failure of the sandwich panel.
These deficiencies of the core may require excessively heavy and expensive skins to reduce bending deflection. Alternately, the overall stiffness of the panel may need to be improved by increasing the thickness or density of the foam core beyond that desired for insulation purposes, which raises the costs of both material and shipping. The relatively low shear modulus of low density plastic foam cores also allows buckling of thin flat panel skins to occur at relatively low stress levels, again calling for overdesign of skins or higher density foam cores as a compensation. Low shear resistance and the absence of reinforcing elements within the foam core further permit the propagation under stress of cracks or fissures between the core and the panel skins as well as within or through the core itself, with resulting deterioration or structural failure of the panel. Still another difficulty is the low compressive strength of most plastic foams, which allows concentrated or impact loads to distort both skins and core.
Reinforcing ribs of metal, wood, fiberglass reinforced plastic and other materials have been used in foam core sandwich panels to mitigate or overcome the structural limitations described above, and foam plastics have been used to fill the voids in parallel-strip sandwich panel cores for additional stiffening. Although both foam core and ribs contribute to the strength of these panels, the structural contribution of the ribs in such constructions is not fundamentally dependent upon the presence of the foam core, and the ribs do not depend on the foam to perform their structural functions.
Various methods of introducing insulating foams into the cells of honeycomb have been used for the purpose of adding higher levels of thermal insulation to the structurally adequate honeycomb core. These include such approaches as applying foaming chemicals to the honeycomb cells, for example, as disclosed in U.S. Pat. No. 4,330,494 which uses foamable thermosetting resin to fill the cells, and pressing slabs of plastic foam into the cells. However, these processes are difficult to perform, limit the types of foams which can be used to fill the cells of the honeycomb uniformly, or require large capital investment in machinery. As a result, such composite cores have enjoyed little use in most sandwich panel applications, and many honeycomb core products are consequently deficient in insulation.
Sandwich panels with skins of metal, wood, fiberglass reinforced plastics and similar durable materials are widely manufactured by two basic processes. In one process, liquid chemicals, commonly of polyisocyanurate formulation, are injected between the skins, after which they react and expand to form a rigid foam which bonds itself to the skins to form the sandwich panel. The other major method of producing sandwich panels is by adhesive lamination wherein preformed panel skins are bonded by adhesive to cores of rigid foam boards or slabs which have been cut from expanded foam billets or wherein uncured resins and reinforcing materials are applied to the surfaces of such foam boards and subsequently cured to form rigid skins.
Sandwich panel laminators use a wide variety of these preformed cores, including polyurethane, polyisocyanurate, extruded polystyrene, expanded polystyrene, polyvinylchloride and foam glass. The use of polyurethane and polyisocyanurate foams in lamination processes is severely restricted in spite of large demand for sandwich panels containing such cores. The cost of such core materials in the form of boards or slabs cut from billets is substantially higher than the cost of chemicals used in foam-in-place processes, a differential of typically two to three times. In addition, since most polyurethane and polyisocyanurate billet stock is manufactured for applications other than sandwich panels, the stock is not usually formulated with physical properties designed primarily for structural sandwich panel use. The best foam stocks having appropriate properties for sandwich panels are produced by very few suppliers and in limited geographic areas.
Polyisocyanurate foam is also produced as board stock with attached facings for use as insulation in roofs and other construction applications. This foam material satisfies common fire performance specifications, is manufactured in large quantities in numerous locations, and is sold as a relatively low priced commodity. In spite of its compelling cost advantages, and the modest structural requirements of most construction panel cores, such insulation has seen very limited use as core material for sandwich panels. Both thickness and flatness of the roof insulation boards have unacceptably wide tolerances for most sandwich panel applications. While planing or sanding the facings eliminates this problem, it also removes the skin part of the foam board having the highest density and strength. Even more serious, available polyisocyanurate foam formulations are not consistently tough enough, and their friability, brittleness and low shear strength can result in serious structural failures. These show up as delamination or foam shear under conditions of structural loading, thermal stress or surface impact.
Plastic foam cores for more structurally demanding sandwich panel applications, such as the hulls of boats, are commonly made of linear or cross-linked polyvinyl formulations, in densities of from 2 to 16 pounds per cubic foot. The high cost of these materials, approximately $2 to $20 per board foot, has prevented their significant use in such major medium to high performance applications as highway trailers and recreational vehicles. A further drawback of the polyvinyl foams and of other thermoplastic foams, such as polystyrene, is serious degradation of their physical properties at temperatures encountered in many transportation environments.